This invention relates to information carriers of the optical type and, more particularly, to a method and apparatus for manufacturing masters for replication of optical discs.
The manufacturing method according to the invention is particularly suitable for the manufacture of masters for the Compact Disc, an optical disc on which audio information is stored as discrete pits along a spiral information track of predetermined pitch. The current standard for the compact digital audio disc calls for a track pitch of 1.6 microns, a track width of 0.6 micron and a pit depth of 0.12 micron, this pit depth providing a sufficient difference in intensity between light reflected from the land and the pits to provide tracking information for the disc player.
In the current process for making masters for the replication of the compact optical disc, a film of chromium or other adhesion promoting material is deposited on a polished glass disc to provide adhesion between the glass surface and a layer of photoresist material. A positive photoresist layer is then applied to the chromium surface by a spinning coater or other suitable means, the thickness of this layer being maintained within a narrow range of 0.11 to 0.13 microns over the recording area of the disc. The photoresist-coated glass blank is then placed on a turntable and while being rotated at a predetermined speed is exposed with a focused laser beam having a spot size of approximately 0.6 micron, with the laser beam moved at a constant feed along the radial direction of the rotating disc. The laser beam is modulated or turned on and off at a predetermined rate by an acousto-optic modulator.
After recording, the exposed photoresist is developed by immersion in a suitable chemical solution to remove the areas that have been exposed to the laser beam. Thus, the developed disc, often called a master, contains a continuous spiral track of discrete pits which correspond to the recorded information. The process thereafter from master to stamper forming is similar to the electroplating process of an ordinary analog LP disc, consisting essentially of coating the developed photoresist with a conductive layer and then subjecting the substrate to nickel electroforming. This nickel replication of the information record is separated from the substrate and used as a stamper to press or mold plastic discs having the same pattern of discrete pits as the developed photoresist.
Because in this prior art process the depth of the pits in the final stamper is controlled by the thickness of the photoresist layer, the thickness of the photoresist must be controlled within a very narrow range around 0.12 micron and, moreover, the process cannot tolerate any defects on the surface of the substrate significantly larger than 0.1 micron, thus putting severe constraints on the manufacturing process. Another disadvantage of this known process is that only one nickel replication can be made from the photoresist surface because the pattern on the photoresist is destroyed in the process of separating the nickel stamper from the photoresist, thus making the process relatively expensive for mass production.
An earlier process capable of producing a more durable master described in U.S. Pat. No. 3,894,179 comprises the steps of coating a disc-shaped glass plate with an etchable silicon oxide layer of predetermined thickness, coating the silicon oxide layer with a thin metal film such as chromium, coating the metal film with photoresist, rotating the disc relative to a focused laser beam, intermittently exposing the photoresist for variable periods which correspond to the information to be recorded, developing the photoresist, and etching the exposed portions of the metal layer and the silicon oxide layer by sputter etching or by means of a photolithographic process. The depth of the pits being determined by the thickness of the silicon oxide layer, this thickness must be controlled to extremely close tolerances, and because the sputtering process preferred for etching is performed in a vacuum, the process is relatively expensive.
Thus, the need exists for an improved method for manufacturing masters for optical discs which will result in a more durable and less costly master than those produced by present methods, and which can be carried out with a minimum of apparatus and attention. Accordingly, the primary object of the present invention is to provide a process for manufacturing an optical disc master that is sufficiently durable to make multiple nickel replications therefrom, thereby to lower the manufacturing cost of optical discs.
Another object is to provide a method for manufacturing optical disc masters which eliminates the disadvantage of the conventional process of requiring extremely close tolerances on the thickness of the photoresist coating.
A corollary object is to provide a process for making optical disc masters which utilizes a photoresist coating as an etch mask so as to greatly relax the tolerances on the thickness and uniformity of the photoresist coating.